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Re: [RFC PATCH 11/28] x86/pvh: Avoid absolute symbol references in .head.text
- To: Ard Biesheuvel <ardb+git@xxxxxxxxxx>, <linux-kernel@xxxxxxxxxxxxxxx>
- From: Jason Andryuk <jason.andryuk@xxxxxxx>
- Date: Wed, 25 Sep 2024 17:10:56 -0400
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- Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>, <x86@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Uros Bizjak <ubizjak@xxxxxxxxx>, Dennis Zhou <dennis@xxxxxxxxxx>, Tejun Heo <tj@xxxxxxxxxx>, Christoph Lameter <cl@xxxxxxxxx>, Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>, "Paolo Bonzini" <pbonzini@xxxxxxxxxx>, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>, Masahiro Yamada <masahiroy@xxxxxxxxxx>, Kees Cook <kees@xxxxxxxxxx>, Nathan Chancellor <nathan@xxxxxxxxxx>, Keith Packard <keithp@xxxxxxxxxx>, Justin Stitt <justinstitt@xxxxxxxxxx>, Josh Poimboeuf <jpoimboe@xxxxxxxxxx>, "Arnaldo Carvalho de Melo" <acme@xxxxxxxxxx>, Namhyung Kim <namhyung@xxxxxxxxxx>, "Jiri Olsa" <jolsa@xxxxxxxxxx>, Ian Rogers <irogers@xxxxxxxxxx>, Adrian Hunter <adrian.hunter@xxxxxxxxx>, Kan Liang <kan.liang@xxxxxxxxxxxxxxx>, <linux-doc@xxxxxxxxxxxxxxx>, <linux-pm@xxxxxxxxxxxxxxx>, <kvm@xxxxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>, <linux-efi@xxxxxxxxxxxxxxx>, <linux-arch@xxxxxxxxxxxxxxx>, <linux-sparse@xxxxxxxxxxxxxxx>, <linux-kbuild@xxxxxxxxxxxxxxx>, <linux-perf-users@xxxxxxxxxxxxxxx>, <rust-for-linux@xxxxxxxxxxxxxxx>, <llvm@xxxxxxxxxxxxxxx>
- Delivery-date: Wed, 25 Sep 2024 21:11:34 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
Hi Ard,
On 2024-09-25 11:01, Ard Biesheuvel wrote:
From: Ard Biesheuvel <ardb@xxxxxxxxxx>
The .head.text section contains code that may execute from a different
address than it was linked at. This is fragile, given that the x86 ABI
can refer to global symbols via absolute or relative references, and the
toolchain assumes that these are interchangeable, which they are not in
this particular case.
In the case of the PVH code, there are some additional complications:
- the absolute references are in 32-bit code, which get emitted with
R_X86_64_32 relocations, and these are not permitted in PIE code;
- the code in question is not actually relocatable: it can only run
correctly from the physical load address specified in the ELF note.
So rewrite the code to only rely on relative symbol references: these
are always 32-bits wide, even in 64-bit code, and are resolved by the
linker at build time.
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
Juergen queued up my patches to make the PVH entry point position
independent (5 commits):
https://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git/log/?h=linux-next
My commit that corresponds to this patch of yours is:
https://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git/commit/?h=linux-next&id=1db29f99edb056d8445876292f53a63459142309
(There are more changes to handle adjusting the page tables.)
Regards,
Jason
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